Conditions such as osteoarthritis, cancer or trauma may cause degeneration of the articular surfaces between the trapezium and the first metacarpal in a hand causing the patient discomfort and sometimes severe pain during thumb circumduction.
Various total replacement prosthesis have been proposed for bones in the human wrist. For example, U.S. Pat. No. 4,936,860 to Swanson, issued Jun. 26, 1990, discloses a metallic total scaphoid replacement implant. Also, U.S. Pat. Nos. 4,955,915 and 4,969,908 both to Swanson, issued Sept. 11, 1990 and Nov. 13, 1990, respectively, disclose total lunate replacement implants. Although not directed to a carpal bone implant, U.S. Pat. No. 4,936,854 to Swanson, issued Jun. 26, 1990, discloses an implant anchored in the radius and having a cupped, or dished, surface for stabilizing the proximal carpal row and preventing ulnar migration thereof.
As an alternative to total carpal bone replacement, it is well known in the art to resurface the distal surface of the trapezium with a prosthetic implant. For example, the Silastic.RTM. Trapezial Implant H.P., manufactured by Dow Corning Wright Corp. 5667 Airline Road, Arlington, Tenn. 38002, is made from medical grade silicone rubber elastomer for use as an interpositional spacer between the trapezium and the first metacarpal joint of the thumb. The implant is provided with a short cylindrical stem extending from its proximal surface which fits into a cavity prepared in the trapezium. The head, or distal articular surface, of the implant is thin and dome-shaped.
One of the goals of implant design is to minimize the production of wear debris particles. Wear particles can never be completely eliminated with presently available implant materials because all moving parts, e.g., implants that articulate against bone, wear to some degree. It is generally believed by some that elastomers generate wear particles at a more accelerated rate than inelastic materials. However, because the distal articular surface of the aforementioned Silastic.RTM. Trapezial Implant H.P. is domed, i.e., not anatomically representative of a natural trapezium, that implant must be manufactured from an elastomer due which has the ability to deform so as to better articulate with existing joint anatomy.